It is essential to confine the operation of the projecting boom or other rotatable and/or pivotable structures of a crane within a predefined range in order to avoid contact with adjacent elements such as electrical wires, buildings or other structures located at the working site.
At present, cam surfaces are fixedly secured to a curb-ring provided on a crane platform disposed adjacent a rotating derrick or the pivoting boom of the crane. The mounting of these cam surfaces is usually accomplished by fixing them by means of bolts which extend through the cam members and which pressed them on the curb-ring. This is time-consuming, especially in cases where it is wished to frequently vary the working range of the movable crane structure.